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Topic: mosfet failure reasons? (Read 3 times) previous topic - next topic

winner10920

Yeah typo, srry
so for that how would I configure my fets?

Grumpy_Mike

Configure? You don't configure components.
I assume you mean wire them up?
What sort of FETs, what sort of voltage do you need to switch, what sort of drive do you have?

Generally you need a p-channel and n-channel, along with a few resistors to ensure that the two are not on at the same time.
Do a google image search for some schematics.

winner10920

How's this link work for a shematic, seem correct to you? http://www.cadvision.com/blanchas/hexfet/npch-sw-sch.htm
im gonna use an irf9520 I think i  and an irf 540
My plan to drive it is from a 555 timer, im gonna set up a dual supply opamp to take the 555 output from 0/12v to -12/+12,

dc42

If what you're trying to do is drive a speaker or similar load with a 5KHz square wave, you might be better off using bipolar transistors instead of mosfets. Use a cmos 555 (which drives the output right to the supply rails, unlike the bipolar version), and replace your N-channel source follower with an NPN small signal transistor driving a PNP power transistor (see http://en.wikipedia.org/wiki/Sziklai_pair but also add a resistor between the base and emitter of the PNP transistor). Then add an opposite pair (PNP small signal transistor driving NPN power transistor) to pull the output down to ground. This isn't quite as efficient as using a mosfet half h-bridge (because you will have maybe 1.5v drop across whichever power transistor is turned on), but it avoids the problems of shoot-through that you tend to get with home-made h-bridges.

If the power required in the load is very large, then the mosfet h-bridge is the most efficient solution; but in this case I recommend you buy a mosfet h-bridge driver chip. You won't need any p-channel mosfets because these driver chips are always designed so that both the upper and lower switches in each half h-bridge are n-channel.
Formal verification of safety-critical software, software development, and electronic design and prototyping. See http://www.eschertech.com. Please do not ask for unpaid help via PM, use the forum.

Grumpy_Mike

I don't like that because it is possible to turn both FETs on at the same time.
You should look for a circuit that can't do that even by accident.

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